1. Field of the Invention
The present invention relates to a speaker system with enhanced efficiency and frequency characteristics. More particularly, the invention relates to a speaker system with an expanded bass register which is accomplished by coupling an acoustic tube to the back side of a speaker unit.
2. Description of the Related Art
A speaker system with an expanded bass register which is accomplished by coupling an acoustic tube to the back side of a speaker unit is known.
The conventional speaker system is described below with reference to FIG. 5. FIG. 5 is a sectional view showing the constitution of a conventional speaker system. In the diagram, numeral 1 denotes a cabinet, 2a, 2b, 2c, 2d and 2e are partitions for forming an acoustic tube by partitioning the inside of the cabinet 1, 3 is an opening window disposed in the lower part of a front panel of the cabinet 1, and 4 is a speaker unit disposed above the front panel.
The partitions 2a, 2b, 2c, 2d and 2e disposed in the cabinet 1 form an acoustic tube in the cabinet 1, and the sound delivered from the back side of the speaker unit 4 is radiated from the opening window 3 through the acoustic tube.
The acoustic tube resonates at a specific frequency fn, and assuming the overall length of the acoustic tube to be L and the sound velocity in the air to be C, the resonance frequency fn is expressed as EQU fn=(C/4L).times.(2n+1)
where n is 0, 1, 2, or another integer.
In the case of n=0, that is, at the frequency resonating with 1/4 wavelength in the acoustic tube, the acoustically magnified sound is radiated from the opening window 3 of the acoustic tube, and the efficiency is notably enhanced near the frequency at the reproduction limit of the bass.
At n=1, that is, at the frequency resonating with a 1/2 wavelength in the acoustic tube, the sound is radiated from the front side of the speaker unit 4, while the sound from the back side is inverted in phase by a delay of a half wavelength in the acoustic tube, and is radiated from the opening window 3. Sound from the front side and the back side are added in the front space of the cabinet 1, so that the sound pressure level increases at that frequency.
At n=2, that is, at the frequency resonating with a 3/4 wavelength in the acoustic tube, as in the case of n=0, the acoustically magnified sound is radiated from the opening window 3.
Thus, in the bass region, a speaker system of high efficiency has been realized.
In such conventional speaker systems, however, even at frequencies in the medium and high frequency region corresponding to n=3 or higher, resonance occurs in the acoustic tube, and the sound is radiated from the opening window 3. The sound from the opening window 3 interferes with the front sound of the speaker unit 4. Peaks and bottoms of sound pressure occur in the medium and high frequency region as indicated by the broken line in the reproduced sound pressure frequency characteristic diagram in FIG. 2. The characteristics of the medium and high pressure region are highly deteriorated resulting in lowered sound quality. In addition, the frequency of the bass reproduction limit is defined by the overall length L of the acoustic tube, and therefore to reproduce the lower frequency region, the overall length L of the acoustic tube is desirably extended, which causes to increase the size of the speaker system.